added 'zbefore_optgroup_event' to allow disable optgroup selection
added 'zbefore_optgroup_event' to allow disable optgroup selection

michaelBenin/zurbfoundation5-multiselect,00F100/zurbfoundation5-multiselect,00F100/zurbfoundation5-multiselect,icyz/zurbfoundation5-multiselect,icyz/zurbfoundation5-multiselect